Calorie Burn Accuracy: Garmin Connect or RunAhead

Hey Guys-

I've been using my Garmin heart rate monitor for about 2 months. I liked the idea of having an accurate calorie burn for my runs. When I upload my info to Garmin Connect and to RunningAhead the 2 calories burns are consistently different. RunningAhead always shows that I burn about 40 more calories or so. Which is more accurate?! How are the getting their calculations?!